1. Field of the Invention
The present invention relates to an inspection method and an inspection apparatus associated with inspecting the external views of the reticles for large scale integrated circuits (LSI) fabrication or the patterns of LSIs themselves.
2. Description of the Related Art
Conventionally, it is a general practice that reticles for LSI fabrication or the patterns of LSIs themselves are inspected by providing two optical systems using metallurgical microscopes, simultaneously observing identical portions of the patterns under test, and obtaining a difference between these portions to detect a defect.
As the LSI patterns have decreased in size, the above-mentioned conventional method can no longer cope with the recent LSI patterns due to the limitation in resolving power. To overcome this problem, a method as disclosed in U.S. Pat. No. 5,572,598 was proposed. In this method, a laser beam having good convergence characteristics is used as the light source. This laser is collected into a microscopic spot. The surface of a test piece, such as a pattern for LSI fabrication, is scanned with this laser beam. An image of the observed surface of the test piece is constructed based on the variation in light quantity of the laser beam transmitted through or reflected from the test piece.
The above-mentioned method, however, uses a technique in which the test piece is scanned with the laser spot in a two-dimensional manner to obtain an observed image. Therefore, as compared with the conventional technique in which an observed image is obtained in a batch by using a camera or an equivalent detector in a single-dimensional or two-dimensional manner, the time for observed image detection increases remarkably.
Moreover, as the resolving power for inspection continues to increase, there is also a drastic increase in data processing requirements. It is therefore strongly desired to shorten the detection time for the observed image of test pieces.
It is therefore an object of the present invention to solve the above-mentioned problems by providing an inspection method and an inspection apparatus that shortens the time for inspecting the reticles for LSI fabrication semiconductor chips, or the patterns of LSIs themselves (hereinafter referred to as xe2x80x9ctest piecesxe2x80x9d) and that have higher precision than the prior art.
In carrying out the invention and according to one aspect thereof, a method of inspecting a high-precision pattern by scanning a surface of a test piece with a laser beam and using at least one of a light beam reflected from the surface of the test piece and a light beam transmitted through the surface of the test piece includes the steps of: branching the laser beam into a plurality of laser beams in order to scan the surface of the test piece with the laser beam as the plurality of branched scan laser beams simultaneously; assigning an identification marker to each of the plurality of branched scan laser beams; and identifying each of the plurality of branched scan laser beams by the identification marker to provide an image of the surface of the test piece corresponding to each of the identified branched scan laser beams.
The laser beam branching step may include a step for splitting the laser beam into two, a step for tilting the optical axis of one of the split laser beams, and a step for synthesizing the two split laser beams. The identification marker may either be a different polarized state assigned to each branched scan laser beam or a variation in light intensity assigned to each branched scan laser beam in a time division manner.
Preferably, the laser beam has an ultraviolet wavelength.
In carrying out the invention and according to another aspect thereof, an apparatus for inspecting a high-precision pattern by scanning a surface of a test piece with a laser beam and using at least one of a light beam reflected from the surface of the test piece and a light beam transmitted through the surface of the test piece may include: a scanning means for scanning the surface of the test piece with the laser beam; a laser beam branching means for branching the laser beam into a plurality of laser beams in order to scan the surface of the test piece with the laser beam as the plurality of branched scan laser beams simultaneously; an identification marker assigning means for assigning an identification marker to each of the plurality of branched scan laser beams; a radiating means for radiating the plurality of branched scan laser beams assigned with the identification markers onto the surface of the test piece; an image signal detecting means for detecting at least one of the light reflected from the surface of the test piece and the light transmitted through the surface of the test piece; a system control having an image processing unit for identifying each of the plurality branched scan laser beams by the identification markers and detecting a defect by obtaining an image of the surface of the test piece by using a detect signal obtained from the image signal detecting means, an image display section for displaying an desired image, and an input section for inputting data from outside; and an XY stage for holding the test piece to drive the same in X-axis and Y-axis directions.
The laser beam branching means may include a splitting means for splitting the laser beam into two, an optical axis changing means for tilting the optical axis of one of the two split laser beams, and a synthesizing means for synthesizing the two split laser beams.
The laser beam branching means may be a plurality of unit laser beam branching means provided in at least one of parallel and series arrangements, the unit laser beam branching means including one splitting means for splitting the laser beam into two, one optical axis changing means for tilting the optical axis of one of the two split laser beams, and one synthesizing means for synthesizing the two split laser beams.
The optical axis changing means may include a wedge-shaped glass plate.
The identification marker to be assigned by the identification marker assigning means may be a different polarized state assigned to each of the plurality of branched scan laser beams or a variation in a light intensity assigned to each of the plurality of branched scan laser beams in a time division manner.
The identification marker assigning means for assigning the variation in light intensity that provides the identification marker may have an ultrasonic modulating means for performing analog modulation on each of the plurality of branched scan laser beams to change a light intensity thereof and a modulation signal generating means for outputting a modulation signal to the ultrasonic modulating means in a predetermined time division manner.
Further, preferably, the laser beam may have an ultraviolet wavelength.
The laser beam radiated from the laser light source is branched into a plurality of laser beams by the laser beam branching means, so that the test piece surface can be scanned with the plurality of laser beams for scanning predetermined ranges of the surface. By combining, in at least one of parallel and series arrangements, a plurality of unit laser beam splitting means for splitting the laser beam into two, tilting the optical axis of one of the split laser beams, and synthesizing the split laser beams, a desired number of branched scan laser beams including an odd number thereof can be generated.
By assigning an identification marker to each of the branched scan laser beams, the branched scan laser beams can be identified in the detect signal obtained from the image signal detecting means for detecting at least one of the light beams reflected from and transmitted through the test piece surface. Consequently, an image of the wide test piece surface can be obtained in a short time for defect detection.
By the XY stage for holding the test piece and driving the same in the X-axis and Y-axis directions relative to the laser beam radiation position, the test piece can be scanned in the X-axis direction. When the test piece has been scanned in the X-axis direction once, the test piece is step-fed in the Y-axis direction to be scanned in the direction opposite to the X-axis direction. This operation is repeated to scan all over the subject area of the test piece in a scan width in which the plurality of branched scan laser beams are arranged side by side.
A first advantage of the present invention is that the inspection method in which the ultrasonic deflector and the technique of splitting the laser beam into two are combined can expand the image signal detection per unit time from conventional 500 points to 1000 points. Further splitting of the laser beam can further increase the number of points per unit time. Consequently, the time for inspecting defects of reticles for LSI fabrication, for example, can be shortened to enhance productivity. This also significantly reduces the cost of LSI itself.
A second advantage of the present invention is that the UV light having wavelength of 363.8 nm is used for the light source, so that, as compared with the prior-art resolution of about 0.3 xcexcm for defect detection, a defect size as small as 0.1 xcexcm can be realized, enabling the defect detection of higher precision patterns than the prior art. This provides an extremely effective inspection technique for the recent LSI fabrication reticles for example that are getting more microscopic in feature.
A third advantage of the present invention is that, compared with the prior-art technique in which the same laser light source is used for both illumination and autofocusing, optical axis adjustment and the like can be made easily and, at the same time, the accuracy of autofocus detection can be enhanced because Hexe2x80x94Ne laser (wavelength 632.8 nm) is used for the autofocusing light source independently of the light source for illumination.
A fourth advantage of the present invention is that highly precise alignment of test piece can be performed by using the stage having 3 degrees, of freedom, i.e. x, y and xcex8 directions.